This invention relates generally to means and methods for obtaining measurements and relates, more particularly, to solid state measurement circuits.
The circuits with which the present invention is to be compared are logarithmic signal processing circuits commonly used in instrumentation applications. These circuits of the prior art commonly utilize a bipolar transistor employing a matched pair of transistors connected for logarithmic current measurement. However, unless means are incorporated within the circuit to compensate for the intrinsic temperature variations of the transistor pair, measurements obtained with the circuit may be inaccurate.
A temperature-sensing resistor (i.e. of a predetermined temperature coefficient) can be utilized within the logarithmic circuit to compensate for the aforementioned intrinsic temperature variations, but the resulting circuit is limited in several respects. For example, the resistors do not possess precise resistance versus temperature characteristics necessary to provide accurate compensation over a relatively broad range of operating temperatures. Furthermore, because the compensating resistors and the log transistor chip may be at slightly different temperatures due, at least in part, to different self-heating rates of the two devices and because the compensation resistor does not correct for variations in the emission coefficient of the log transistor pair, temperature compensation is imperfect. Moreover, the temperature coefficient of the compensating resistor has a poor tolerance rendering the circuit not compatible with high-volume production techniques. Still further, the prior art circuit is further limited in that it is not compatible with the possible use of a small thermoelectric cooler to cool the log transistor pair for the purpose of reducing measurement errors due to leakage current and extending the useful measurement range well below 1 pA. Still further, the prior art temperature compensation means normally require precise scale factor calibration of the log conversion function and thus require the use of a manually trimmed resistor locally in the log converter circuit rather than the often preferred approach of computer-assisted scale factor correlation after the measurements have been digitized.
It is an object of the present invention to provide a new and improved circuit and an associated method for logarithmically measuring current which circumvents the aforementioned limitations associated with circuits of the prior art which employ temperature compensating resistors.
Another object of the present invention is to provide such a circuit which provides an output signal which is totally independent of temperature.
Still another object of the present invention is to provide such a circuit which possesses a high degree of accuracy over a relatively large decade range of input signal.
Yet another object of the present invention is to provide such a circuit which is uncomplicated in construction and effective in operation.